1. Field of the Invention
The present invention relates to the fabrication of metal staircases.
2. Description of the Related Art
In commercial and industrial buildings, such as multi-story office buildings or multi-story factory buildings, the floors may be constructed of reinforced concrete. The distance from one floor to the next is often not uniform, so that the stairs are either a steel stairway, fabricated off the construction site, i.e., "off-site", or a concrete stairway, formed at the construction site, i.e., "on-site".
As used herein, the term "staircase" means the structure containing a stairs; "stairs" means a series of steps in a unitary structure; and "stairway" means one or more flights of staircases usually with connective landings. The present invention is particularly adapted for, and the illustrations are related to, a set of steps forming straight stairs between two adjoining floors. However, it may be utilized in the formation of stairways having landings.
Metal stairs are partially built in a fabrication plant, away from the construction site. Because the distance between floors varies from one building to another, and even within a single building, each metal staircase must be specially fabricated.
Basically, there are two types of metal staircases in commercial use, namely, bolted staircases and welded staircases. A widely used type of welded staircase uses stringers (elongated side members) to which brackets (carriers) are welded, generally in the steel fabrication off-site shop. One type of bracket uses a continuous ribbon for each stringer which is bent at right angles to form the supports for the stairs. The stairs are separately fabricated, off-site, with each stair comprising a pan (horizontal portion) and a riser (vertical portion). The complete metal stairway is assembled on-site with the stair pan being individually welded onto the stringer brackets. The assembly, prior to welding, must be done carefully, by skilled iron workers, who clamp the stringers and pans to correctly align them ("square up") before they are welded. If the staircase is incorrectly aligned, the welds must be ground away and re-welded. The welding should be performed by skilled workers as many of the welds are visible to those using the staircase. One series of welds is used to connect the riser to its adjacent tread. Even though, for appearance, those welds are made from the back of the riser, the heat of the weld causes distortion of the metal. That metal distortion is highlighted by the paint coating and detracts from the appearance of the staircase.
The alternative construction, of bolted metal staircases, is also widely used. In that construction, the stringers are pre-fabricated, off-site, by welding brackets to the stringers. The brackets have pre-formed holes to receive bolts. On site, the stringers are held in upright position and then the floor pans are placed in their positions one after the other, resting on the brackets and bolted down. Frequently the floor pans are forced into position by hammer blows as the previously positioned floor pans may draw the stringers together. At that time the staircase is mounted by the workers who are bolting it together, i.e., they build up from the bottom stair to the top stair. Each stair pan is aligned with its holes matching the holes of the bracket and the prior stair pan, using a guidance pin. In addition to the bolts at the sides, joining the stair pans with the brackets, additional bolts are required to join each stair pan with the riser of its adjoining stair pan. A typical staircase may require about 100-150 bolts, each of which must be correctly tightened to its nut. Some of the bolts are very difficult to reach, especially for a man doing the job alone. For example, the top bolts require two men, one on a ladder or else one man reaching over the stair from above. The bolting together of the stairway is time-consuming due to the location and number of the bolts. If the staircase is misaligned, frequently many bolts must be loosened, the staircase re-adjusted and the bolts re-tightened. Sometimes bolts, and their nuts, are lost or misplaced. Even so, mistakes are quicker and easier to correct in a bolted staircase than with a welded staircase. The force of the bolts, when they are being tightened, is generally sufficient to straighten the stringers and otherwise correctly align (square-up) the staircase.
After the metal staircase, either welded or bolted, is assembled, on-site, its stair pans are filled with concrete. The concrete will hide some, but not all, of the welds and bolts.
U.S. Pat. No. 4,285,177 entitled "Reinforced Tread Assembly" shows the use of glass fiber reinforced tread assemblies in a metal stairway system. U.S. Pat. Nos. 3,498,012 and 3,606,236 show a metal stairway system using prefabricated stair treads and locking devices to lock the stair treads to risers.